The present invention is related to metal deposition processes as used for instance for the formation of conductive patterns connecting active or passive devices as well as integrated circuits. Particularly, such conductive patterns can be formed at least partly by means of an electroless deposition technique.
Currently, copper is being introduced in ULSI metallization schemes as a replacement for aluminum due to its lower resistivity and better electromigration resistance. Copper electroplating is the most popular deposition technique today. To avoid contamination of the surrounding insulating layers and/or the substrate, copper is mostly deposited on a Cu diffusion barrier layer. However, to allow electroplating, first a conductive seed layer has to be formed on top of the barrier layer(s) in order to get reliable electroplated copper deposition. Usually, a sputtered copper layer is used for this purpose. However, for dual damascene type processing with very high aspect ratio openings in dielectric layers such as trenches, via holes and contact holes, the step coverage of sputtered (classical or by means of an Ion Metal Plasma) barrier layers and Cu seed layers is expected to become the limiting factor for subsequent filling with e.g. electroplated copper. As a consequence, alternative deposition routes for copper seed layer formation can be attractive for future device technologies. Electroless copper deposition has the potential of becoming a viable alternative because it can deliver high step coverage depositions at a very low cost. The principle of electroless metal deposition is based on the generation of electrons at a catalytically active or an activated surface in contact with a solution of metal ions in the presence of a suitable sacrificial electron donor. These electrons are capable of reducing the metal ions leading to the deposition of the metal on the activated surface. Because this process does not occur on non-activated layers, the resulting deposition technique is inherently selective. Furthermore, in principle, it should be easy to integrate such a process in currently available copper electroplating tools (which are already on the market or will be so in the near future), provided amongst others that the electroless plating solution is stable at room temperature for at least about two weeks and the process margins of the plating solution are not too tight. Nowadays most electroless copper plating solutions do not meet these specifications. They have often a limited stability and can only be effectively used in a limited pH range which makes them very sensitive for slight variations in the composition of the plating solution as such variations result in small variations in the pH but often lead to a large decrease in the deposition rate.
Moreover, most available electroless copper plating baths do not fulfill the stringent requirements for copper plating in sub-micron high aspect ratio features onto the typical Cu diffusion barrier layers used in ULSI processing. Typical barrier layers are Ti, TiN, Ta, WNx, TaN, Co and any combination thereof, and other Cu diffusion barrier layers known in the art. One of the problems of electroless copper deposition on barrier layers and particularly on e.g. TiN is the evolution of copious amounts of hydrogen gas which is detrimental for the quality of the copper layer formed because it leads to severe blistering of the copper layer. The use of cyanide as a hydrogen suppressor, as practiced in many commercial copper plating solutions, is not acceptable in this business due to safety issues. Another problem related to the state-of-the-art plating solutions is the bad adhesion of electroless deposited copper on such barrier layers.
Moreover, most electroless copper plating solution compositions are based on salts containing mainly sodium as the counterion. These high levels of sodium ions in the plating solutions can introduce severe reliability problems, particularly when sodium reaches the semiconductor device junctions, as this is known to be a production yield killer in semiconductor device manufacturing. Therefore as a further requirement the level of sodium ions in the plating solution should be very limited or negligible.
Nowadays, electroless copper plating solutions often use EDTA as the complexing agent and formaldehyde as the reducing agent. The complexing agent is required to keep Cu(II) ions in solution at the relatively high pH values at which formaldehyde operates as a reducing agent. Recently, the trend is to move away from strong complexants such as EDTA. Due to their strong complexing power for many metal ions, more stringent requirements for the environment are expected for plating solutions based on strong complexing agents. Consequently, there is a need for other more environmentally acceptable complexing agents.
It is an aim of the invention to provide a plating solution for electroless copper deposition which is substantially sodium free (low level sodium) and which comprises an environmentally friendly complexing agent. The plating solution should have a long life-time after make-up (thermal stability) of at least two weeks at room temperature and should be easy to replenish to keep the plating characteristics well within specifications over the plating periods. Furthermore, slight variations in the composition of the plating solution may cause only small variations in the Cu deposition rate on an activated surface of a substrate immersed in the plating solution.
It is another aim of the invention to provide a method to form Cu-containing layers on an activated surface of a substrate by means of an electroless deposition technique using the plating solution of the present invention. This plating solution and deposition method should be such that the formation of hydrogen gas during the plating period is avoided or severely limited. Particularly, this method has to result in a sufficiently high deposition rate at low temperatures.
A further aim of the invention is to provide a Cu seed layer on a diffusion barrier layer, said diffusion barrier layer may act simultaneously as a wetting layer. Particularly when a Cu-containing layer is deposited on a barrier layer, the Cu-containing layer has to be formed with good adhesion on the diffusion barrier layer. Seed layers have typically a thickness below 300 nm.
Still a further aim of the invention is to provide a Cu plating solution suitable to form a relatively thick Cu-containing layer on a barrier layer or on a seed layer. This is particularly useful to fill up openings with high aspect ratios in insulating layers such as via holes, trenches and contact holes like e.g. in damascene architectures. The Cu-containing layers formed have typically a thickness between 200 nm and 2 xcexcm.
The present invention is related to the fabrication of at least a part of a Cu-containing layer or a Cu-containing pattern used for the electrical connection of active or passive devices as well as integrated circuits. Such Cu-containing patterns and/or layers can be formed on an activated surface of a substrate by means of immersion of said substrate in an electroless plating solution. Therefore, in an aspect of the present invention, an aqueous solution for electroless deposition of Cu on a substrate is disclosed, said solution comprising:
a source of copper Cu (II) ions;
a reducing agent;
an additive to adjust the pH of said aqueous solution to a predetermined value; and a chemical compound for complexing said Cu ions, said chemical compound having at least one part with chemical structure COOR1xe2x80x94COHR2 (as can be seen in FIG. 2a)), R1 being a first organic group covalently bound to the carboxylate group (COO), R2 being either hydrogen or a second organic group. Examples of such first or second organic groups are hydrocarbon groups, while for instance the chemical compound for complexing the Cu ions can be selected from the group consisting of diethyltartrate, diisopropyltartrate and dimethyltartrate. The pH of the plating solution ranges typically from 11 or 11.5 to 13.5, while the temperature at which the solution can be applied ranges from 10 to 50 degrees C., or 45 degrees C. or below, or from room temperature to 40 degrees C. Examples of a reducing agent are formaldehyde, paraformaldehyde, hydrazine, amine boranes, alkali metal borohydrides, alkali metal hypophosphites or a derivative of one of the aforementioned reducing agents.
In an embodiment of the invention, a Cu plating solution is disclosed based on an organic based complexing agent, wherein the ratio between the concentration of said source of copper Cu (II) ions and the concentration of said complexing agent in said solution is in the range from 1/5 to 5/1 or from 1/10 to 10/1 or from 1/25 to 25/1.
In another embodiment of the invention, a Cu plating solution is disclosed wherein the complexing agent is a chemical compound with chemical structure COOR1xe2x80x94CHOHxe2x80x94CHOHxe2x80x94COOR1, R1 being an organic group covalently bound to the carboxylate group (COO). For instance, hydrocarbon groups can be used as organic groups. Particularly, a chemical compound selected from the group consisting of diethyltartrate, diisopropyltartrate and dimethyltartrate can be selected.
In another aspect of the invention, a method is disclosed for forming a Cu-containing layer on a substrate comprising the steps of:
preparing an aqueous solution comprising a source of copper Cu (II) ions, a reducing agent, a chemical compound for complexing said Cu (II) ions, said chemical compound having at least one part with chemical structure COOR1xe2x80x94COHR2, R1 being an organic group covalently bound to the carboxylate group (COO), R2 being either hydrogen or an organic group, and an additive to adjust the pH of the solution to a predetermined value;
immersing said substrate in said aqueous solution for a predetermined period to thereby form said Cu-containing layer at least on an activated surface of said substrate. For instance, this Cu-containing layer can be formed on a Cu diffusion barrier layer formed on the substrate.
In an embodiment, a method for filling an opening in an insulating layer is disclosed, wherein, after forming at least one opening in an insulating layer formed on a substrate, a barrier layer can be formed on at least one inner wall of said opening. Examples of such openings are via holes, contact holes, and trenches. Examples of such barrier layers are layers of Ti, or TiN, or Ta, or tungsten nitride, or TaN, or Co or a combination thereof. Particularly this barrier layer can also act as a wetting layer like e.g. a TiN layer with a Ti layer thereon. A Cu-containing metal, i.e. an alloy of or pure Cu, is deposited using the electroless plating solution of the present invention. This electroless deposition can be performed in a chamber of an electrodeposition tool. The electroless deposition can be performed in at least one deposition step to thereby completely fill the openings. Alternatively, first a thin Cu-containing seed layer can be formed using the electroless deposition method of the present invention, thereafter a second Cu-containing metal can be deposited on said seed layer using a different Cu deposition technique as e.g. electroplating of Cu to thereby completely fill the openings.